How much force of Friction?

Question

A girl pushes a box 300N at 2 m/s. She stops pushing the box and it comes to rest 3 seconds later. How large is the force of friction on the box?

A. 20 N
B. 50 N
C. 150 N
D. 200 N
E. 300 N

Answer 1

First calculate the acceleration by dividing the lost velocity (2 m/s) by the time during which it changed (3 s). You also need to determine the mass of the box by dividing its weight (300 N) by the acceleration due to gravity (9.8 m/s^2). Then, use F = ma, where you have calculated the mass m and the acceleration a, to calculate the force.

Answer 2

Are we simply doing your homework for you?

Firstly, assume that no other forces are acting on the box. You say that she is pushing a box with a given force and that the box is not accelerating (it is travelling at a constant 2m/s). This is important as it means that she is pushing it with exactly the same force that friction is applying in the opposite direction. When the box is being pushed, therefore, the friction is -300N (note that this is a MINUS force i.e. a force in the opposite direction). When she suddenly stops pushing the box, the box will slow down:

F = ma

The acceleration must be negative as the box comes to rest, so the force (still negative) will decrease. In the whole situation, the maximum force of friction is -300N but there is no option for that from your answers! I can only assume that your teacher who set this doesn't know a lot about physics and is saying the answer is E (this force would actually be in the same direction as that which the girl is pushing and consequently the box would accelerate away from the girl!). Maybe something is missing in this question, but from the answers you have given I would have to say it would be E: 300N

But this is wrong. The maximum magnitude force applied by the friction is 300N, but this is not the maximum force.

Ask your teacher to explain to you what magnitude of force is. Actually, explain it to your teacher.

Answer 3

Have to assume the push force is 300N, in which case that is also the Ff, Answer E.

If the box WEIGHS 300 N, then david's right and the anwer is A.

The ? is ambiguously worded: "box of 300 N weight" or "pushes with a force of 300 N against a box"......? Could be either way as stated.

Answer 4

V = Vo + a*t
where V = final velocity = 0, Vo = original velocity = 2 m/s, and you need to find a = acceleration. (This will come out negative, meaning it's deceleration.)

For the next part, you're going to need the mass of the box:
W = m*g
where W = the box's weight, m = the box's mass, and g = acceleration due to gravity = 9.8 m/s^2.

Then Ff = m*a
where Ff = force of friction, m = the mass of the box, and a = accleration. Ff will probably be negative, that's means it exerts its effect in the direction opposite the original velocity.